Paper presentation on -
NANOSCALE REVERSIBLE MASS TRANSPORT FOR ARCHIVAL MEMORY
-by Aditya P. Jaiswal Vaibhav S. Gaikwad & Sushrut C. Holey
ABSTRACT
In today’s world data storage is of vital importance. But present data storage devices are reliable but cannot compete with today’s requirement of data storage. They have short life period, say from 5 yrs to 50 yrs and memory devices consume large space. Nanoscale memory devices can be best example in Nanotechnology for future resolutions in memory devices. A simple electromechanical memory device which is used to storage of trillion data at a high density with a long lifetime. It consist of an iron nano-particle shuttle is controllably positioned within a hollow nano-tube channel made up of carbon, which is also called as “CARBON NANO-TUBE”.
The shuttle can be moved reversibly via an electrical write signal and can be positioned with nano-scale precision. The position of the shuttle can be read out directly via a blind resistance read measurement, allowing application as a nonvolatile memory element with potentially hundreds of memory states per device. The shuttle memory has application for archival memory storage, with information density as high as 1012 bits/in2, and thermodynamic stability in excess of one billion years.
OBJECTIVES
The main aim of discovery of nano tube is to storage the trillion-bit memory with high density for billions of years. Although truly archival storage is a global property of an entire memory system, the first inescapable requirement for such a system is that the underlying mechanism of information storage for individual bits must exhibit a persistence time much longer than the envisioned lifetime of the resulting device. A single bit lifetime in excess of a billion years demonstrates that this system has the potential to store information stably for any practical desired archival time scale. Thus, nano scale electromechanical memory devices such as those described here present a new solution to ultrahigh density, archival data storage.
Introduction
Digital storage devices have become ubiquitous in our lives; music, photographs, and even the written word have moved from their traditional analog formats to newer digital ones. However, this move to digital storage has raised concerns about the lifetime of storage media. While ancient analog archival media (e.g. stone and vellum) can preserve their data for thousands of years, digital storage technologies such as optical discs, magnetic discs, and magnetic tape are thought to last at most a century (and in many cases much less). Obviously stone and vellum are not well-suited to today’s volume of data: the pits in a CD are spaced about a micron apart, carvings in stone have feature sizes closer to a centimeter. New archival technologies will have to combine the best features of both, storing data at a high density with long lifetimes.
CONSTRUCTION
Scientist has developed a new mechanism for digital memory storage with the potential to store data with both long lifetime and high density. This memory device consists of a crystalline iron nano-particle enclosed in a multi-walled carbon nano-tube. The nano-tube can be reversibly moved through the nano-tube by applying a low voltage, "writing" the device to a binary state represented by the position of the nano-particle. The state of the device can then be subsequently read by a simple resistance measurement. The ever-growing demand for digital storage of videos, images, music and text calls for storage media that pack increasingly more data onto chips that keep shrinking in size. However, this demand runs in sharp contrast to the history of data storage. Compare the stone carvings in the Egyptian temple of Karnak, which store approximately two bits of data per square inch but can still be read after nearly 4,000 years, to a modern DVD which can store 100 giga (billion)
bits of data per square inch but will probably remain readable for no more than 30 years. This illustration shows the configuration of a new digital memory storage device consisting of an iron nano-particle shuttle that moves through a carbon nano-tube when a voltage is applied. This memory device can pack a trillion bits of data into one square inch of medium and retain that data for a billion years. Zettle and his collaborators were able to buck data storage history by creating a programmable memory system that is based on a moveable part - an iron nano-particle, approximately 1/50,000th the width of a human hair, that in the presence of a low voltage electrical current can be shuttled back and forth inside a hollow carbon nano tube with remarkable precision. The shuttle’s position inside the tube can be read out directly via a simple measurement of electrical resistance, allowing the shuttle to function as a nonvolatile memory element with potentially hundreds of binary memory states.
The shuttle memory has application for archival data storage with information density as high as one trillion bits per square inch and thermodynamic stability in excess of one billion years. Furthermore, as the system is naturally hermetically sealed, it provides its own protection against environmental contamination. The low voltage electrical write/read capabilities of the memory element in this an electro-mechanical device facilitates large-scale integration and should make for easy incorporation into today’s silicon processing systems. Zettle believes the technology could be on the market within the next two years and its impact should be significant.
The shuttle memory has application for archival data storage with information density as high as one trillion bits per square inch and thermodynamic stability in excess of one billion years. Furthermore, as the system is naturally hermetically sealed, it provides its own protection against environmental contamination. The low voltage electrical write/read capabilities of the memory element in this an electro-mechanical device facilitates large-scale integration and should make for easy incorporation into today’s silicon processing systems. Zettle believes the technology could be on the market within the next two years and its impact should be significant.
The nano-scale electromechanical memory device can write/read data based on the position of an iron nano-particle in a carbon nano-tube. The memory devices here are displaying a binary sequence 1 0 1 1 0. Although truly archival storage is a global property of an entire memory system, the first requirement is that the underlying mechanism of information storage for individual bits must exhibit a persistence time much longer than the envisioned lifetime of the resulting device. A single bit lifetime in excess of a billion years demonstrates that this system has the potential to store information reliably for any practical desired archival time scale.
IMPORTANCE OF CONSTITUENTS IN NANO-TUBE
The multi-walled carbon nano-tube and enclosed iron nano-particle shuttle were synthesized in a single step via pyrolysis of ferrocene in argon gas at a temperature of 1,000 degrees Celsius. The nano-tube memory elements were then ultrasonically dispersed in iso-propanol and deposited on a substrate. A transmission electron microscope provided high-resolution imaging in real time while the memory device was in operation. In laboratory tests, this device met all the essential requirements for digital memory storage including the ability to overwrite old data.
NANO-TUBE INTRODUCTION
The miniaturization of nonvolation silicon-based memory devices has revolutionized the creation, access, and exchange of digital information. There are tremendous benefits in continuing this miniaturization into the nano-scale. Unfortunately, a general trend for memory storage, independent of medium, is decreasing lifetime with increasing memory density. Conventional digital memory configurations in use today, with densities of order 10-100 Gbits/in2, have an estimated lifetime of only 10-30 years.
WORKING
Figure 1a below shows a schematic of our memory device. The primary element is a nano particle encapsulated within a multi wall carbon nano tube. The nano particle can shuttle back and forth within the nano tube channel and the physical location of the particle defines the logic state. The key challenges to physically realizing and operating the shuttle memory of Figure 1a (given on next page) are reliable construction of the device, a means to “write” to it (i.e., translate the shuttle), and a means to nondestructively “read” it. Furthermore, for the memory to be useful, long-term stability with overwriting must be possible. Studies of electro migration-driven mass transport on or in nano tubes suggest that transition metal nano particles may be good candidates for such a shuttle. We synthesize the required heterogeneous nanostructure consisting of a nano tube with an enclosed iron nano particle shuttle in a single step via pyrolysis of ferrocene in argon at 1000 °C. The nano tube memory elements are then ultrasonically dispersed in isopropanol and deposited on a substrate. For diagnostic purposes, we use a custom fabricated silicon nitride membrane compatible with transmission electron microscopy (TEM) as a substrate.
Figure 1b-f shows a series of transmission
electron microscopy (TEM) images of the nano tube with an enclosed shuttle. The electrical contacts at the ends of the nano tube are outside the field of view. Upon application of an electrical current through the nano tube, the shuttle moves via electro migration forces. TEM observation indicates that the iron nano particle is crystalline throughout the shuttling process. As the current direction is reversed, the direction of motion for the shuttle is reversed. Dropping the current to zero rapidly quenches the shuttle motion,”freezing” its position. Although mass transport on or inside nano tubes has been previously described for a variety of metals, this is, to our knowledge, the first demonstration of precision controllable, fully reversible, long-range solid state mass transport within a nano tube. The speed of the shuttle can be tune over several orders of magnitude by varying the applied bias voltage V, as shown in Figure 1g. Just beyond a threshold for the onset of motion at V ~ 1.55 V, the shuttle moves at a rate of ∼1 nm/s, on the order of the speed of slow continental drift,14 while at V ) 1.75 V, the highest bias applied to this device, the shuttle moves at 1.4 μm/s, comparable to the speed of the motor protein myosin.15 With other devices we have observed that the shuttle velocity can be increased by at least 4 orders of magnitude beyond this, exceeding 2.5 cm/s, the maximum speed we can detect at high magnification due to the frame rate of our TEM video camera. The true top speed could be considerably higher. In addition to continuously translating the shuttle, the shuttle motion can be precisely “stepped”. Application of a short (~20 ns) voltage pulse on the order of 2 V causes the shuttle to translate 3 nm. Successive pulses induce successive translations analogous to, but an order of magnitude more precise than, the walking motion of myosin VI, which moves in 36 nm steps along an acting filament. The combination of static biasing and pulsing allows precision control of the shuttle position from the micro- to the nano scale. We note that the shuttle motion is dictated strictly by the applied dc bias, irrespective of the presence or absence of the TEM imaging electron beam. The different shuttle position states, as shown in Figure 2b, clearly serve as digital memory. If we define positions left of image center as “0” and right of image center as “1”, then a desired logic state can be written by application of a voltage pulse, of suitable polarity and sufficient duration, which physically positions the shuttle. The written state can be read out via TEM imaging, as shown in Figure 2a. While interesting from a diagnostic point of view, using TEM to read out the state of the device is clearly impractical. Most desirable would be a simple two-terminal electrical readout. We find that the axial electrical resistance of the nano tube is sensitive to the physical location of the enclosed nano particle shuttle. In Figure 2b, we show the nano tube resistance R as the shuttle is continuously repositioned. Although some noise is apparent, Figure 2a demonstrates that R tracks the TEM-determined position coordinate remarkably well. With the discrimination similar to that used for TEM positioning, the logic states are thus faithfully read out from blind resistance measurements alone. Importantly, probing the state of the system via small voltage pulses is nonperturbative and does not alter the shuttle position; hence the memory electrical readout is nondestructive. This is demonstrated in Figure 3, where the states 101010 have been sequentially written to the device and read out in succession four times for each written state. Both the TEM-determined position state and the resistively determined state are in agreement, with no state destruction cause by readout. Due to the two-terminal configuration, such memory devices could be scaled to produce memory densities ~1 Tbit/in2, greater than current state of the art hard storage, with memory density ~200 Gbit/in2. The shuttle memory density could be increased by another order of magnitude or more by applying multiple thresholds to store many states per device. The unique geometry afforded by the encapsulated shuttle naturally yields a hermetically sealed system, immune to environmental contamination. In contrast to conventional memories, which may suffer from interaction between magnetic domains as well as material breakdown, the information in the shuttle memory is only compromised if the shuttle moves. We now turn to an examination of the readout mechanism and the expected lifetime of the memory device and written state. The readout mechanism is tied to the electrical resistance of a multi wall nano tube, which is itself a subject of much speculation and controversy. Our experiments demonstrate that the iron shuttle, despite residing entirely within the core of the nano tube, serves as a position-dependent scattering center. In a model of on-tube transport with inter shell coupling, the shuttle could locally alter inter shell coupling as well as influence quantum interference of electron wave functions along the tube. If the electronic transport is diffusive, the system may be in part modeled as regions of materials with different receptivity. Position dependence in the total resistance could then come from spatial variations in the receptivity of the nano tube (caused by defects) or simply by geometric effects. If the transport is in the ballistic regime over limited segments, local resistance changes could be due to electron resonance effects.
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